C−C and C−H Bond Activation at Ruthenium(II): The Stepwise Degradation of a Neopentyl Ligand to a Trimethylenemethane Ligand

Abstract

Ruthenacyclobutane complexes (SiP3)(PMe3)Ru(CH2EMe2CH2) (SiP3 = MeSi(CH2PMe2)3; 1, E = C; 2, E = Si) were synthesized from (SiP3)(PMe3)RuCl2 (3) and 2 equiv of the Grignard reagents, Me3ECH2MgCl. Metallacycle 1 was found to reversibly interconvert with the allyl complex (SiP3)Ru(Me)(η3-CH2CMeCH2) (4) and PMe3 when heated above 75 °C. From the results of kinetic studies and thermolysis of labeled material, the interconversion is proposed to take place by reversible β-methyl elimination/insertion. Conversion of 1 to 4 is an endothermic process (ΔH° = 14.3 ± 1.1 kcal mol-1), but it is entropically favorable (ΔS° = 40.9 ± 2.8 cal K-1 mol-1) due to the loss of the PMe3 ligand. Activation parameters for the β-insertion were determined to be ΔH⧧ = 26.0 ± 1.2 kcal mol-1 and ΔS⧧ = −10.5 ± 0.9 cal K-1 mol-1. Allyl complex 4 has been isolated as a mixture of isomers (7:1 endo:exo). The mechanism of interconversion of 4endo and 4exo was determined by 1H{31P} NMR spectroscopy (EXSY) to be a process involving a stereoc...